You can built an Ipsec VPN tunnel between Cisco routers, both on Dynamic IP addresses
In order to configure a LAN-to-LAN Virtual Private Network (VPN) tunnel between two routers with dynamic IP addresses, complete these steps apart from the basic configuration:
Configure the set peer dynamic command on one side of the tunnel with the use of the static crypto map.
On the remote router, configure the dynamic crypto map without the use of the peer statement.
With the use of the set peer dynamic command, the host name of the IP Security (IPsec) peer is resolved through a domain name server (DNS) lookup before the router establishes the IPsec tunnel.